Optical fiber amplifiers and lasers have rapidly become important components of optical communications systems. Optical fiber amplifiers are used to intensify optical signals that are attenuated along the fiber-optic communication path. They have replaced cumbersome electrical repeaters in fiber-optic communication links allowing true all-fiber optical communications systems to be realized. Similarly, optical fiber lasers have been proposed to generate an optical carrier for fiber-optic communications systems. These lasers can be externally modulated or mode locked, and in some cases are an alternative to diode lasers as sources of high-power light in fiber optic communications systems.
Both fiber amplifiers and lasers operate on similar principles. The silica glass in the guided-wave portion of the optical fiber is doped with ions of rare-earth element such as, for example, erbium. The energy structure of the erbium ions is such that signal light with wavelengths of approximately 1530-1565 nm can be amplified in the fiber if the population of the excited states of the erbium ions is such that the rate of stimulated emission exceeds that of spontaneous emission and absorption. In such a circumstance, light within the gain bandwidth entering the optical fiber will experience net gain, and will exit the fiber with greater power. If a mechanism is established to recirculate this amplified signal in the fiber, for example, by placing the appropriate reflectors at the ends of the fiber, then laser action can occur in the fiber if the net gain equals the loss of the light within some optical bandwidth. In either case, it is crucial to excite the erbium ions into the proper excited state for gain to occur. This can be accomplished by exciting (pumping) the erbium ions with light near wavelengths of 980 nm, which is most conveniently provided by a high-power diode laser that is coupled into the guided-wave portion of the optical fiber. The relatively small cross-sectional area of this portion helps to ensure high intensity and therefore allows appreciable gain of the signal wavelengths. However, those skilled in the art will appreciate that the properties of the signal produced by such an amplifier or laser will depend to a large extent on the properties of the diode laser used to pump the fiber itself.
In a practical system, the diode lasers are permanently and robustly connected with an opto-mechanical apparatus to a length of undoped optical fiber which in turn is connected to the doped fiber in the optical amplifier or laser. The assembly consisting of the diode laser, opto-mechanical apparatus and optical fiber is called a pigtailed diode laser. Presently, many pigtailed diode lasers have undesirable characteristics such as wavelength and intensity instabilities that create noise in the pumped system. The most troublesome sources of diode laser noise in the 980 nm diode lasers are mode-hopping noise and wavelength fluctuations that are caused by unwanted variable optical feedback into the diode laser or changes in temperature or injection current. The noise is especially detrimental in fiber amplifiers because it increases errors in the amplified optical communication signal and detracts from the practicality of these devices.
There are many techniques to reduce the effect of such diode laser noise. An example is an active electrical system that detects the variation in output of the fiber amplifier caused by a fluctuation in the diode laser characteristics and feeds back a signal into the laser diode at the correct phase to reduce the laser fluctuation. Unfortunately, this technique adds cost and complexity to the amplifier. It is preferable to employ a passive method of reducing diode laser fluctuations. An attractive solution is to feed back into the pump diode laser a portion of its own light. These lasers are very sensitive to optical feedback, and if such feedback is properly controlled, improved laser operation can result. Feedback is usually provided by an external reflector such as a mirror or diffraction grating, and external optical elements such as lenses are required to manipulate and guide the light out of and back into the diode laser cavity. Although the external optics and reflectors can often be quite compact, it is difficult and expensive to align such a system, and the mechanical and thermal stability can often be inadequate for use in fiber amplifiers and lasers. A more rugged technique for control of diode laser characteristics is required.